A transmission is an apparatus through which power and torque can be transmitted from a vehicle's power unit to a load-bearing device such as a drive axis. Conventional transmissions include a variety of gears, shafts, and clutches that transmit torque through the transmission at finite, stepped gear ratios.
A continuously variable transmission is a different type of transmission that can include an infinite number of gear ratios. The arrangement of gears and the like of a continuously variable transmission can improve the fuel efficiency of the vehicle by enabling the power unit to operate at its most efficient revolutions per minute (RPM) for a range of vehicle speeds.
A continuously variable transmission can have multiple operating modes such that each operating mode covers a portion of the overall ratio spread of the transmission. Each operating mode is selectable, e.g., by a clutch that is engaged by the application of hydraulic fluid pressure as commanded by the transmission's control unit. Some continuously variable transmissions have a “geared neutral” mode, in which the continuous variation of ratio passes through the geared neutral mode in transitioning from a reverse ratio to a forward ratio. In the geared neutral position, the vehicle's speed is zero, independently of the rotational output speed by the vehicle's drive unit. Transmissions that have a geared neutral mode may be referred to as infinitely variable transmissions.
Like the continuously variable transmission, an infinitely variable transmission can advantageously improve a vehicle's fuel efficiency, reduce emissions, and provide enhanced control. Infinitely variable transmissions can be included in applications such as tractors, snowmobiles, heavy off-highway construction, mining equipment, and marine applications. However, there are physical limitations with conventional infinitely variable transmissions that restrict some on-highway applications such as buses. Many conventional infinitely variable transmissions have gearing configurations that cause the transmission to be too long, for example, to fit within a space positioned near the rear end of a bus. Infinitely variable transmissions therefore have had limited penetration in the on-highway market.
In addition, an infinitely variable transmission can include a variator assembly for transferring torque therethrough. The variator assembly is designed to rotate in a single direction. In some aspects, an infinitely variable transmission can be coupled to a diesel engine. The output of a diesel engine provides input torque to the transmission by rotating an input shaft, torque converter, or other transmission input device. Diesel engines are known to suddenly kick back, or rotate, in a direction opposite from its normal operating direction during engine shutdown. Since the variator assembly can only rotate in one direction, the sudden kick back can cause mechanical damage to the variator assembly. To avoid this condition and protect the variator assembly, conventional infinitely variable transmissions include a one-way clutch connected to ground. Thus, as the engine begins to turn backwards, the clutch prevents this energy from being transferred to the transmission. Instead, the inertia is returned to the engine.
The problem with the one-way clutch setup is that the inertia returned to the engine can damage or impact the performance of the engine. Also, at start-up, there is concern about potential torsional vibrations affecting the transmission.
Thus, a need exists for a gearing configuration of a variator-inclusive transmission that reduces the overall transmission length and can be operably coupled to a powered vehicle. Further, there is a need to reduce torsional vibrations during startup and reduce the amount of inertia returned to the engine during shutdown.